Signaling system



Jam. Z39 3945,. H. M. PRUDEN SIGNALING SYSTEM 9 sheets-sheet 1 FiledAug. 19, 1945 Im 239 w45. H, M, UDEN 367,792

SIGNALING SYSTEM Filed Aug. 19, 1943 9 Sheets-Sheet 2 /A/ l/E/VTR H M.'PRUDEN A .TTOPNEV JM. 23? @945. H, M PRUDEN v 2,367,792

SIGNALING SYSTEM Filed Aug. 19, 1943 9 Sheets-Sheet 3 I :lc/VAL1mm-MITTE@ v /NVENTOR H M PRUDEN By @MM ATTORNEY Jan. 239 E945., H. M.PRUDEN SIGNALING SYSTEM Filed Aug. 19, 1943 9 Sheets-Sheet 4 laHl'ATTORNEY Jan 23, E945. H. M. .PRUDEN SIGNALING SYSTEM Filed Aug. '19,1943 9 Sl'leets--Sheetl 5 /NVENTOR H M PRI/DEN Wd@ WM A-'TRNEY 'www2Jam., 23, 3945. H. M. PRUDEN SIGNALING SYSTEM Filed Aug. 19, 1945 9Sheets-Sheet 5 /NVE/vroR H M PRUDEN ATTORNEY 5mm 23, E945.

H. M. PRUDEN SIGNALING SYSTEM Filed Aug. 19; 1943 9 Sheets-Sheet 7/A/VE/VTOR HMP/PUDEN ATTORNEY Jam. 23, E945. H. M. PRUDEN SIGNALINGSYSTEM Fled Aug. 19, 1943 9 Sheets-Sheet 8 mmm /Nl/E/v Tof? H. M PRUDE NArron/VEV Jan. 23, 1945. H. M. PRUDEN SIGNALING SYSTEM 9 sheets-'sheet 9Filed Aug'.

/N VEN TUR H. M. PRUDEN Qi? da Patented Jan. 23, 1945 anni UNITED STATESPATENT OFFICE SIGNALING SYSTEM Harold M. Pruden, Maplewood, N. J.,assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application August 1'9, 1943, Serial No.499,201

9 Claims. (Cl. 179-27) This invention relates to signaling systems andparticularly to systems in which voice frequency currents are used forthe transmission of signals.

Objects of the invention are the provision of more reliable signalreceiving means in systems employing voice frequency signals andparticularly the receiving of signals of a character which minimize thepossibility of false operations in response to speech or otherinterference currents.

This invention is a signal receiving means adapted to respond to signalseach of which consists of a plurality of current impulses in succession,each current impulse ci a signal being of a different one of threesignaling frequencies.

Seizure, selective, ringing and disconnect signals are .received oncalls incoming over the trunk or line with which the signal receivingmeans is associated. Supervisory off-hook and oli-hook signals anddisconnect acknowledgement signals are received on calls outgoing overthe trunk or line with which the signal receiving means is associated. Aseizure signal received on an incoming Call and an off-hook supervisorysignal received on an outgoing call consist of an impulse of current ofone of the signaling frequencies followed immediately by an impulse ofcurrent of another of the signaling frequencies. Selective signalscorresponding to a train of dial impulses received on an incoming callconsist of an impulse of current of the third of the signalingfrequencies for the duration of each dial impulse and an impulse ofcurrent of said other of the frequencies :for the duration of theinterval between successive dial impulses of a train, the currentimpulses representing the first dial impulse being immediately precededby a preparatory or enablement signal which is similar to a seizuresignal. A disconnect signal received on an incoming call consists cf theenablement signal followed by an impulse of current of the third of thesignaling frequencies for a predetermined interval; and, if disconnectis not effected during receipt of the current of the third frequency,this impulse is followed by alternate impulses of current of the irstand second of the signal frequencies. An on-hook supervisory signal received on an outgoing call -consists of an impulse of current of each ofthe rst, second and third of the signaling frequencies in succession. Adisconnect acknowledgement signal received on an incoming call consistsof alternate impulses of current cf the first and second of thesignaling frequencies.

A clear and complete explanation of the invention will be facilitated byconsidering a system in which the invention and its various features areembodied, one such system being represented schematically in thedrawings whichform apart of this specification. The invention is notlimited in its application to the particular system rdisclosed in thedrawing but is generally applicable to -any signaling system in which itis desirable to employ voice frequency currents for the transmission ofsignals.

Referring to the drawings:

Figs. 1 to 4 show, in a first toll cnice, an operators position OP, anoutgoing trunk circuit O'Il, a two-way trunk circuit TWTI, an incomingtrunk circuit ITI, a toll Aline circuit TLC i, a signal transmitter STIand a signal receiver SR!y all individually associated with an intertolltrunk line TLl2 extending to a second toll o-fce;

Figs. 5 to 9 show, in the second toll ofce, a toll line circuit TLC2,signal receiver SR2, signal transmitter ST2, two-way trunk circuit TWT2,outgoing trunk circuit 0T2, anda toll route selector circuit TS2, allindividually associated with the inter-toll trunk line TLIZ;

Fig. 9 further shows an incoming trunk circuit IT2 terminating at anoperators position, and shows an outgoing trunk circuit OTS, two-waytrunk circuit TWTS, toll route selector TSB and toll line circuit TLC3associated with another intertoll trunk line TL23; and

Fig. 10 shows the relative position in which Figs. 1 to 9 are to beplaced to form an operative arrangement.

The system represented in the drawings ncludes a plurality of tolloiiices each of which comprises a toll board with operators positionsand cords for answering calls incoming from local manual and dialoiices, from toll subscribers lines, and from other toll oilices and forextending incoming calls to switching trunks leading to called localmanual and dial oices, to intertoll trunks and to called tollsubscribers lines. Outgoing jacks are directly connected to trunks tolocal oiiices, to toll subscribers lines and to intertoll trunks.Answering jacks are directly connected to recording trunks, to tollsubscribers lines, to intertoll trunks andto incoming trunks terminatingin the banks of toll route selectors. Toll route selectors are providedfor use on incoming calls to establish connections with incoming trunkcircuits and with trunks to other toll oces and to called local oices.

Reference may be had to the patent to R. E. King et al., No. 2,209,777granted July 30, i940, for a detailed description of the cord `andoperators position circuits provided in each of the toll offices forinterconnecting calling and called lines and trunks and controlling thecompletion of toll calls. The outgoing trunk circuits OTI, T2 and OT3,incoming trunk circuits ITI and IT2, two-way trunk circuits TWTI, TWTZand TWT3 and the toll route selector circuits TS2 and TS3 are al1similar to corresponding circuits of the aforementioned King et al.patent. The toll line circuits TLCI and TL2 and the associated signalreceiving circuits SRI and SR2 are arranged for voice frequencysignaling in accordance with applicants invention and the associatedsignal transmitting circuits STI and ST2 are arranged for voicefrequency signaling according to the invention described in theapplica-tion of J. G. Walsh, Serial No. 499,200, filed on even dateherewith. The toll line circuit TLC3 andthe associated signaltransmitting and signal receiving circuits may be arranged for compositesigt naling as shown in the aforementioned King et al. patentor may bearranged for voice frequency signaling.

The toll line circuit TLC I comprises a repeating coil RCI, adirectionally selective coil 200, a cut-off relay 204 and an outgoingsignal transformer 209. The cut-ofi relay 204 is normally operated toclose a talking connection between the repeating coil RCI and thetalking conductors of the outgoing and two-way trunk circuits OTI andTWTI. When released, relay 204 connects the outgoing signal transformer209, through windings of coil 200, to the repeating coil RCI; andconnects the filter unit 490 of the associated signal receiver SRI totheinput transformer 400 of signal receiver SRI. The toll line circuit TLCZis similar to the toll line circuit TLCI, the reference charactersapplied to like elements having the same tens and units digits.

The signal transmitter STI comprises a signal receiving relay 300controlled by relay |09 of outgoing trunk circuit OTI on outgoing callsand controlled by relay |29 of two-way trunk circuit TWTI on incomingcalls. The transmitter STI further comprises sources GI, G2 and G3 ofsignaling currents of frequencies FI, F2 and F3 respectively, and aplurality of control relays of desired time characteristics to effectthe transmission of the required signaling currents. The signaltransmitter ST2 is similar to the transmitter STI, except that the threesources of .signaling current G4, G and G6 are of frequencies F4, F5 andF6 respectively. The reference characters applied to correspondingelements of signal transmitters STI and ST2 have the same tens and unitsdigits.

The signal receiver SR2 comprises an incoming signal transformer 500,ampliers 50| and 508, filters 5|0, 520 and 530 tuned to pass current offrequencies FI, F2 and F3 respectively, rectifiers 5| I, 52| and 53| andrelays 5|4, 524 and 534 for response to signal impulses of frequenciesFI, F2 and F3 respectively. The signal receiver SR2 further comprisesauxiliary signal receiving relays SIB, 526, 550, 555 and 560 forrepeating the received signals in a direct current signaling circuit.The signal receiver SR2 further comprises a filter network 590consisting of three sections 59|, 592 and 593 respectively, tuned to thefrequencies F4, F5 and F6, to prevent outgoing signals from interferingwith the response of relays 5I4, l524 and 534 to incoming signals. Thesignal receiver SRI is similar to the receiver SR2, except that it isarranged to respond to signal vcurrents of frequencies F4, F5 and FE andis arranged to prevent outgoing signals of frequencies FI, F2 and F3from interfering with response to incoming signals. The referencecharacters applied to corresponding elements of signal re ceivers SRIand SR2 have the same tens and units digits.

The three frequencies FI, F2 and F3 used for signaling in one directionover toll line TL|2 differ from the three frequencies F4, F5 and F6 usedforsignaling in the other direction over toll line TLIZ. For instancethe sources GI, G2 and G3 associated with signal transmitter STI maytransmit current of 600 cycles, M300-cycles and 1400 cycles,respectively; in which case the filters 5|0, 520 and 530 of signalreceiver SR2 are tuned t0 pass currents of 600, 1600 and 1400 cycles,respectively. And the sources G4, G5 and G6 associated with signaltransmitter ST2 may transmit current of 1900 cycles, 900 cycles and 1100cycles, respectively; in which case the filters 4|0, 420 and 430 ofsignal receiver SRI are tuned to pass currents of 1900, 900 and 1100cycles, respectively.

Assume now that the operator at position OP in the iirst toll oftice hasanswered an incoming call with the plug API of cord CDI and that theplug CPI is inserted in jack J l to extend the call over toll line TLI2to the second toil office, relays |04, |06 and |09 of outgoing trunkcircuit OTI being thereby operated in the manner described in theaforementioned King et al. patent. The operation of relay |09 closes acircuit through conductor ||0 for operating relay 300 of signaltransmitter STI; and the operation of relay |06 connects ground toconductor |08. The operation of relay 300 opens the normally closedcircuit for operating the cut-off relay 204 of line circuit TLCI, thiscircuit being traced from battery through the inner lower baci: contactof relay 300, back contacts of relays 3I0, 220 and 250, winding of relay204, conductor 203, to ground at the back contact of relay 426 of signalreceiver SRI. The release of relay 204 connects the right winding ofsignal transformer 209 to the left windings of repeating coil RCI andconnects filter unit 490 to input transformer 400. 'The operation ofrelay 300 also closes a circuit including the inner upper back contactoi relay 300 for operating relay 315 and closes a circuit including theinner upper back contact of relay 360 and the upper back contact ofrelay 3.50 for operating relay 350. The operation of relay 300 alsocloses a circuit from source GI of signaling current of frequency FIthrough the lowermost back contact of relay 3I0, inner upper frontcontact of relay 300, left contact of relay 325, inner upper backcontact of relay 2|0, and left winding of transformer 209 to ground; andas soon as the cut-off relay 204 has released, signal current offrequency FI :Ps transmitted through transformer 209, back contacts ofrelay 204, directionally selective coils 200 and repeating coil RCI,over toll line TLIZ to the signal receiver SR2 in the second tolloiiice. The aforementioned operation of relay 300 also opens thenormally closed circuit through the lower winding of relay 320, thedeenergization of this winding and actuation of the contacts of relay320 being delayed for about .020 second by current charging condenser32|. The operation of relay 320 opens the normally closed circuitthrough the upper winding of relay 330, the deenergization of thiswinding and actuation of the contacts of relay 330 being delayed forabout .020 Second by current charging condenser 322. The

operation of relay 320 also closes a circuit through the lower windingof relay 325 to effect the immediate operation of relay 325. Theoperation of relay 325 disconnects source GI from, and connects sourceG2 to, the left winding of transformer 209 thereby'ending thetransmission of current of frequency F| and initiating the transmissionof current of frequency F2 over the toll line TL|2 to the signalreceiver SR2. When relay 330 operates, it closes a circuit including aback contact of relay 350 for operating relay 3|0. Relay 3|0 locksthrough its inner lower front contact, conductor 233, upper back contactof relay 235, and back contact of relay 2M to ground at a back contactof relay 290.

The aforementioned operation of relay 315 causes the successiveoperation of relays 210, 21| and 22D. Relay 210 is slow in operating sothat the operation of relay 220 occurs about .050 second after theoperation of relay 300 and after the operation of relay 3h0. Theoperation of each of relays 2li! and 220 connects ground throughconductor 36| and the inner upper back contact of relay 360 to conductor30| in parallel with the connection of ground to conductor 30| throughthe outer lower front contact of relay 300. The operation of relay 220also closes a circuit from ground through a back contact of relay 240,conductor 24|, uppermost contact of relay 300, conductor 302, uppermostfront contact of relay 220, inner lower back contact of relay 240,conductor 242, a front contact of relay 3|0 and the lower winding ofrelay 320. The energization of the lower winding of relay 320 restoresthe contacts of relay 320 to normal, thereby causing the release ofrelays 325 and 330, the release of relay 330 being delayed by currentcharging condenser 322. The release of relay 325 disconnects thesignaling current source G2 from the left winding of transformer 205 toend the current of frequency F2. The aforementioned operation of relay220 also closes a circuit for operating relay 350; and relay 3530 locksin parallel with relay 380 under the control of relays 230, 29| and 250.The operation of relay 350 opens the operating circuit of relay 3|0;disconnects conductor 35| from conductor 3M, connects ground to thearmature of relay 305; and closes a circuit for reoperating the cut-offrelay 200, this circuit being traced through conductor 205, upper backcontact of relay 250, lowi ermost front contact of relay 220, innerupper front contact of relay 3 l0, inner lower front contact of relay350 and the inner lower front contact of relay 300. also connects groundto conductor 352 thereby causing the energization of both windings 305and the energization of the lower winding of relay 390. With bothwindings energized, the contacts of relay 385 remain in normal position,as shown in the drawings. The energization of the lower winding of relay390 causes the operation of relay 330. The aforementioned connection ofground to conductor 302 also causes energizaton of the winding of relay340 but relay 390 opens the circuit for operating relay 340 to preventits operation at this time. Thus a seizure signal consisting of currentof an impulse of current of frequency Fl for about .020 second followedimmediately by an impulse of current of frequency F2 for about .030second is transmitted respon-- sive to the seizure of outgoing trunkcircuit OT! to effect the operation `of the signal receiver SR2 ashereinafter described.

The seizure signal thus transmitted over toll The operation of relay350line TLI'2 responsive to seizure of trunk'circuit OTI is furthertransmitted through repeating coil RC2, directionally selective coils600, conductors 60| and 602, transformer 500, amplier 50|, transformer505, volume limiting amplifier 508 and transformer 509 to the input sideof each of lters 5|0, 520 and 530. The impulse of current of frequency.liI is further transmitted through filter 5|0 and rectier 5|| to effectthe operative energization of the upper winding of relay 5|4. Theoperationy of relay 5l'4 opens the normally closed short-circuit acrossthe upper winding of relay 550 and closes a circuit for discharging thenormally charged condenser 5|5 thereby to energize the lower, operatingwinding of relay 5|6 in the same direction as its biasing winding. Whenthe impulse of current of frequency FI ends, relay 5|4 releases andcondenser 5|5 is: again charged in series With the lower Winding o-frelay SI5, the direction of the charging current being such that relay5|6 is operated for a predetermined interval of time. The operation ofrelay 5|6 opens the circuit through resistor 5H thereby to decrease thecul'- rent through the lower biasing winding of relay 524 to renderrelay 524 operatively responsive to current of frequency F2. Theoperation of relay 5|6 connects battery through resistor 5|'| toconductor 553; .but neither of relays 550 and 555 vis operativelyaffected since the upper winding of relay 550 is short-circuited throughthe back contact of relay 5|4 and the winding of relay 555 isshort-circuited through the back contact of relay 550. The impulse ofcurrent of frequency F2 immediately following the impulse of current offrequency FI elfects the operation of relay 524. The operation of relay524 opens the short-circuit across the middle winding of relay 526, tocause the operation of relay 526, and connects ground to conductor 525to operate relay 823 of two-way trunk circuit TWT-2. The operation ofrelay 526 causes the release of cutoff relay 604 of line circuit TLC2and closes a circuit through the middle winding of relay 5|6 to holdrelay 5|6 operated. The drop in potential through resistor 566 isthereby increased so that the current through resistor 566 and thebiasing windings of relays 534 and 550 is decreased. 'Ihe aforementionedcircuit for operating relay 823 is traced from conductor 525 through aback contact of relay 550, conductor 565, a back contact of relay 90B ofoutgoing trunk circuit 0T2, conductor 007, back contact of relay 824 oftwo-way trunk circuit TWT2, back contact of relay 821 and the winding ofrelay 823. The operation of relay 823 connects ground to conductor 003to prevent seizure of the trunk circuit TWTZ by any of the selectorssuch as TSB having access thereto. The operation of relay 823 alsocloses a circuit for operating relay 825 in series with the line relay35'! of selector TS2; this circuit is traced from ground at the frontcontact of relay 524, through conductor 525, back contact of relay 560,conductor 555, back contact of relay 906, conductor 907, back contact ofrelay 824, winding of rel-ay 825, a front contact of relay 823 andconductor SI5, then in simplex through both windings of retard coil 9|4,conductors 9|| and 9|2, back contacts o-f relay 920, conductors 92| andS22, front contacts of relay 823, conductors and 852, back contacts ofrelay 853 of selector TS2, resistors 055 and 856, another back contactof relay 853 and through the winding of line relay 857. The operation ofrelay- 825 closes a circuit for operating the slowto-release relay 821and closes a circuit through resistor 826 for holding relay 823 afterrelay 821 operates. The operation of relay 821 opens the operatingcircuit .and closes a holding circuit through resistor 826 for relay823. The operation of relay 821 connects ground to conductor 830 tooperatively energize the lower winding of relay 690 of signaltransmitter ST2 and to hold the connection which is extended through theselector TS2; and this ground is further connected through a backcontact of relay 905 of outgoing trunk circuit T2 and conductor 56| tothe winding of relay 560 of signal receiver SR2 to effect the operationof relay 560. The operation of relay 550 disconnects conductor 564 fromground at the left contact of relay 534, connects this ground toconductor 555 to hold relays 825 and 851 operated, connects ground fromthe front contact of relay 524 to conductor 563 to energize the lowerwinding of relay 526, and connects ground to conductor 562. When theimpulse of current oi' frequency F2 ends, relay 524 releases; but relay56S is held operated until a disconnect signal causes the disconnection`of ground. from conductor 830. as hereinafter described. The release ofrelay 524 causes the delayed release of relay 525, the delay beingintroduced by current charging condenser 521; and the release of relay525` causes the. release of relay 5|5. The

aforementioned operation of relay 851 of selector TS2 closes a circuitfor operating the slow-torelease relay 858. Relay 858 connects ground toan alarm circuit to effect the operation of an alarm in case theselective operations of selector TS2 are not completed within apredetermined interval of time. Relay 850. also connects ground toconductor 830 in parallel with the ground connected thereto at the lowerfront contact of relay 821. The aforementioned operation of relay 690prepares a locking circuit for relay 620, closes a circuit forIoperating relay 69 I, and closes a circuit for operating relay 550. Thecircuit for operating relay 650 is traced from battery at the outerupper front contact of relay 690, winding of relay 650, a back contactof relay 640, inner lower back contact of relay 6304, conductor 562, toground at a front contact of relay 550. The operation of relay 650causes the operation of rela-y 680. With relays A59|), 69| and 650Operated,

the locking ground for relays 1|0 and 160 is supplied at the outer lowerfront contact of relair 550 instead of through back contacts of relays690 and 69 When the calling operator dials the digits of the toll routecode, and the local oflce code and called subscribers number in case thecalled subscribers line terminates in a dial oilice, relay |09 ofoutgoing trunk circuit OTI is alternately released and reoperated inresponse. to each dial impulse to effect a corresponding release andreoperation of relay 300 of signal transmitter STI. The releasev ofrelay 300 responsive to the first dial impulse effects the transmissionof an enablement signal, which is similar to a seizure signal, followedby an impulse of current of frequency F3 which impulse represents thedial impulse. The enablement signal is effective to prepare the signalreceiver SR2 at the other end of the line TLIZ for response to the trainof dial impulses corresponding to the digit dialed. The release of relay300 opens the circuit through the winding of the cut-off relay 204to-cause the release of relay 204, and connects the signal currentsource GI to transformer 209 toV transmit current of frequency FI overtoll line TL|2, this connection being traced from source GI, throughconductor 3|5, lowermost front contact of relay 3|0, upper back contactof realy 300, left contact of relay 325, conductor 2| I, back contact ofrelay 2 I 0 and the left winding of transformer 209. The release ofrelay 300 also opens the above-described circuit through the lowerwinding of relay 320, the operation of relay 320 being delayed for about.020 second by current charging condenser 32|. When relay 320 operates,it causes the immediate operation of relay 325 and the delayed operationof relay 330. The operation of relay 325 disconnects source Gl from, andconnects source G2 to, conductor 2|| and the left winding of transformer209 thereby to end the transmission of current of frequency FI andinitiate the transmission of current of frequency F2 over toll lineTLI2. "I'he release of relay 300 also disconnects ground from conductor30| thereby to open a short-circuit across the winding of relay 380 andthus cause the operation of relay 380 in a circuit traced from batterythrough the winding of relay 315, resistor 316, lower front contact ofrelay 315, winding of relay 380, back contact of relay 385, to ground ata front contact of relay 360. Relay 315 is held operated by the currentin this circuit. The circuit path through the winding of relay 315,resistor 316 and resistor 31| aids in holding relay 315. The operationof relay 380 opens the connection between conductor 30| and resistor316, closes a circuit for operating relay 260, closes a circuit forenergizing the upper winding of relay 390 to cause the release of relay390, closes a circuit for operating relay 310, and

opens the circuit through the upper winding ofl relay 385. The operationof relay 260 prevents the reoperation of relay 204 while a train of dialimpulses is being transmitted. The release of relay 390 closes a circuitfor operating relay 340. The operation of relay 310 opens the circuitpath through resistor 31|, and connects the Winding of relay 350 toconductor 36| so as to hold relay 350 under the joint control of relays310 and 380 after the operation of relay 340.

The energization of the upper winding of relay 385 is maintained bycurrent charging condenser 313 for about .040 second after relay 385operates, the energization of the lower winding of relay 385 thereuponbecoming effective to operate relay 385. The operation of relay 385causes the release of relays 315 and 380. The release of relay 315short-circuits resistor 316 and the release of relay 380 reconnects thewinding of relay 315 to conductor 30| awaiting the reoperation of relay300. The release of relay 380 also opens the circuit through the upperwinding of relay 390 and causes the release of relay 310. Being slow inreleasing, relay 260 remains operated until all of the impulses in thetrain have been received. The energization of the upper winding of relay380 is maintained after relay 380 releases by current charging condenser393; and therefore relay 390 does not operate during dialing unless thedial speed is low. If relay 390 reoperates while relay 315 is released,relay 340 is held operated in a circuit traced through the lowermostfront contact of relay 350, the front contact of relay 340, upper backcontact of relay 315 and conductor 362 to ground at a front contact ofrelay 350. With relays 310 and 360 released and relay 340 operated, thecircuit for operating relay 350 is opened and relay 350 releases. Therelease of relay 350 disconnects source G2 from, and connects source G3to, the left winding of transformer 209 thereby ending the transmissionof current of frequency F2 and initiating the transmission of current offrequency F3 over toll line TLI2. The connection between source G3 andtransformer 209 is traced through the uppermost back contact of relay240, a front contact of relay 220, conductor 352, lower back contact ofrelay 350, front contact of relay 325, conductor 2l I, and back contactof relay 2I0. The release of relay 310 closes the circuit for energizingthe upper winding of relay 385, thereby causing the release of relay385. Relay 390 reoperates in case the circuit through its upper windingis not again closed before the current charging condenser 393 decaysbelow the Value required for maintaining the energization of thiswinding. When relay 300 reoperates at the end of the first dial impulse,it again connects ground to conductor 30| thereby causing thereoperation of relay 315. 'Ihe reoperation of relay 300 also reclosesthe circuit for energizing the lower winding of relay 320 to effect theimmediate release of relay 320. The release of relay 320 closes thecircuit for energizing the upper winding of relay 330 thereby causingthe immediate release of relay 330. The release of relay 320 also opensthe circuit through the lower winding of relay 325 but energization ofthis winding is maintained by current discharging condenser 354, thedischarge path including the inner upper back contact of relay 350 and afront contact of relay 3I0, this current being effective to hold relay325 operated for about .100 second.

The aforementioned operation of relay 315 opens the locking circuit forrelay 340 but relay 340 is held operated through the back Contact ofrelay 390 unless the dial impulse rate is low; in which case theoperation of relay 390 causes the release of relay 340 and the releaseof relay 340 closes the circuit for operating relay 350, thereby to endthe transmission of current of frequency F3 by disconnecting source G3from transformer 209 and to initiate the transmission of frequency F2 byconnecting source G2 to transformer 209. If, however, the dial is not aslow dial, relay 390 does not reoperate since the release of relay 300responsive to the next dial impulse of the train causes the reoperationof relay 380; and relay 380 recloses the circuit through the upperwinding of relay 390 to maintain the energization of this winding andthereby prevent the reoperation of relay 39.0. The reoperation of relay380 also causes the reoperation of relay 350 thereby disconnectingsource G3 from transformer 209 to end the transmission of current offrequency F3 and reconnecting source G2 to transformer 209 to initiatethe transmission of current of frequency F2. The reoperation of relay350 closes a circuit including the lower winding of relay 325 andcondenser 353 whereby the operative energization of this winding ismaintained by current discharging condenser 353. The release of vrelay300 responsive to the second dial impulse of the train also opens thecircuit through the lower winding of relay 320, the operation of relay320 being delayed for about .020 second by current charging condenser32|. The operation of` relay 320 again connects ground to conductor' 326to maintain the operative energization of the-'lower winding of relay325. The operation of relay320 also opens the circuit through the upperwinding of relay 330, the operation of relay 330 being delayed bycurrent 'charging condenser 322. The reoperation of relay 380 responsiveto the second dial impulse received by relay 300 also causes thereoperation of .relay 310; and the reoperation of relay 310 opens thecircuit through the upper winding of relay 385, the deenergization ofthe upper winding and operation of relay 385 being delayed by currentcharging condenser 313. The operation of relay 385 causes the release ofrelays 315, 380, 310 and 350. The release of relay 350 disconnectssource G2 from, and connectsl source G3 to, transformer 209 thereby toend the transmission of current of frequency F2 and initiate thetransmission of current of frequency F3. When relay 300 reoperates atthe end of the second dial impulse, relay 315 is reoperated, relays 320and 330 are released, the transmission of current of frequency F3 isended, and the transmission of current of frequency F2 is initiated inthe manner above set forth as resulting from the reoperation of relay300 at the end of the first dial impulse. Thus an impulse of current offrequency F3 is transmitted for an interval equal to the length of eachdial impulse as indicated by the release of relay 300 and an impulse offrequency F2 is transmitted between succeeding dial impulses except thatin the case of slow dials the transmission of current of frequency F3 isended and transmission of current of frequency F2 is initiated by thereoperation of relay 350 when relay 340 releases due to the operation ofrelay 390. When relay 300 operates at the end of the last impulse of atrain, relay 315 is reoperated, relay 390 operates, relay 340 releasesand relay 350 reoperates; and current of frequency F2 is transmitteduntil relay 325 releases, the release of relay 325 occurring about .1second after the reoperation of relay 350, at which time the dischargecurrent from condenser 353 is no longer effective to maintain theoperative energization of the lower winding of relay 325. Since relay380 remains normal, relay 260 releases thereby reclosing the circuit foroperating the cut-off relay 204.

When a train of dial impulses is transmitted over toll line TLI 2 to thesignal receiver SR2, relay 5I4 is operated responsive to the impulse ofcurrent of frequency FI relay 524 is operated responsive to the impulseof current of frequency F2, and then relays 534 and 524 are alternatelyoperated responsive to the alternate impulse of current of frequency F3and F2 representing each dial impulse of the train. The operation ofrelay 5I4 responsive to the impulse of current of frequency FI causesthe energization of the lower winding of relay 5I6 by currentdischarging condenser 5I5, this current being in the biasing direction.The release of relay 5I4 at the end oi the impulse of current offrequency FI causes the operative energization of the lower winding ofrelay 5I6 by current charging condenser 5I5. The operation of relay 524responsive to the impulse of current of frequency F2 opens theshort-circuit across the middle winding of relay 526 and connects groundthrough conductor 525, a front contact of relay 560, to conductor 563 tooperatively energize the lower Winding of relay 526. The operation ofrelay 526 closes a circuit for energizing the middle winding of relay5I6 to hold relay 5I6 operated after the current charging condenser 5 I5 is insucient to maintain the energization of the lower winding ofrelay The operation of relay 526 disconnects ground from conductor 603to cause the release of 4 operates, it disconnects ground from conductor565, thereby causing the release of relay 825 of two-way trunk circuitand the release of line relay 851 of selector TS2. The operation ofrelay 534 connects ground to conductor 563 to maintain the operativeenergization of the lower winding of relay 526. When relay 534 releasesat the end of the rst impulse of frequency F3, it connects ground toconductor 565 to reoperate relays 825 and 851 and disconnects groundfrom conductor 563, the energization of the lower winding of relay 526being maintained by current charging condenser 521. When relay 524reoperates responsive to the impulse of current frequency F2 followingthe impulse of current of frequency F3, it opens the short-circuitacross the middle winding of relay 526 and connects ground to conductor563 to maintain the operation of relay 526. The alternate operation ofrelays 534 and 524, responsive to alternate impulses of current offrequency F3 and F2 representing succeeding dial impulses of the train,effects the alternate disconnection and reconnection of ground toconductor 565 to alternately release and reoperate relays 825 and 851and maintains the operative energization of relay 526. The release ofrelay 524, at the end of the impulse of current of frequency F2following the impulse of current of frequency F3 representing the lastdial impulse of the train, closes the short-circuit across the middlewinding of relay 526 and disconnects .ground from conductor 563 therelease of relay 526 being delayed by current charging condenser 521.When relay 526 releases it reconnects ground to conductor 603 to effectthe reoperation of cutoi relay 604.

Each release of relay 825 opens the circuit through the winding of relay821 but relay 821 is slow in releasing and remains operated during theresponse of relay 825 to dial impulses. The release of relay 851, inresponse to the rst dial impulse, closes a circuit for operating thevertical stepping magnet 86| and relay 863 in series. The operation ofstepping magnet 86| raises the brushes 81|, 812, 813 and 814 up to therst level of the bank. VON are actuated as soon as the shaft on whichthe brushes are mounted is moved out of normal position. The operationof relay 863 and actuation of springs VON close a circuit for operatingrelay 865, this circuit including the lower contact of springs VON, thefront contact of relay 863 and the outer lower front contact of relay858. Relay 865 closes a locking circuit which includes the back Contactof the rotary stepping magnet 862, the upper front contact of relay 865and the lowermost back contact of relay 853. When relay 851 reoperatesat the end of the rst. impulse, the stepping magnet 86| releases, butrelay 863 is slow in releasing and remains operated until all of theimpulses in the train have been received by relay 851. Relay 858 is alsoslow in releasing and remains operated during the response of relay 851to dial impulse. Each succeeding release of relay 851 eiiects thereoperation of stepping magnet 86| thereby stepping the brushes up tothe level corresponding to the rst digit of the toll route code. Whenrelay 851 remains operated at the end of the last impulse of the train,relay 863 re- The vertical off-normal springs leases closing a circuitthrough the lower front contact of relay 865 and back contact of relay863, for operating the rotary stepping magnet 862. The operation ofstepping magnet 862 advances the brushes into engagement with the firstset of terminals in the selected level and causes the release of relay865. The release of relay 865 causes the release of stepping magnet 862.If the trunk connected to the first set of terminals is busy, theterminal engaged by test brush 813 is marked by a ground potentialconnected thereto; and this ground potential is connected through brush813, a back contact of relay 853, back contact of stepping magnet 862,lower contact of springs VON, to the winding of relay 865. Relay 865 isthereby reoperated to again close the circuit for operating steppingmagnet 862 so as to advance the brushes to the next set of terminals.When the brushes engage a set of terminals connected to an idle trunk,the terminal engaged by brush 813 is not marked by ground potential andrelay 853 is operated by the current in a circuit traced from batterythrough the winding of relay 865, lower contact of springs VON, backcontact of stepping magnet 862, winding of relay 853, lower contact ofthe eleventh-rotary-step springs 859, to grounded conductor 830. Beingmarginal. relay 865 does not reoperate in series with relay 853. Theoperation of relay 853 disconnects conductors and 852 from the windingof relay 851 and connects these conductors through front contacts ofrelay 853 and brushes 81| and 812 to the line conductors of the selectedtrunk or selected auxiliary selector circuit in case more than one stageof toll route selectors are provided. The operation of relay 853 causesthe successive release of relays 851 and 858, the operation of relay 853being maintained as long as relay 821 of twoway trunk circuit TWT2connects ground to conductor 830. Additional trains of impulses receivedby the signal receiver SR2 are repeated over conductor 565 to relay 825of two-wayv trunk circuit TWT2 and to the line relay of any trunk orselector to which the connection is extended. Assume that the call inquestion is one to be completed by an operator in the second tolloflice. In this case the selector TS2 selects a set of terminals such asthe terminals 816 which are connected by conductors |00I, |002, |003 and|004 to an incoming trunk circuit IT2, The operation of relay 853extends the connection from conductors 85| and 852 through brushes 81|and 812, conductors |00l and |002 to effect the operation of the linerelay |005 of trunk circuit IT2 thereby to light an answering lamp inusual man ner. When the plug of a cord is inserted in jack J I0 toanswer the calll the answering lamp is extinguished and arelay |034 isoperated in the manner set forth in the aforementioned King et al.patent. Relay |034 connects ground to conductor |004, through brush 814,a front contact of relay 853 of selector TS2, a back contact ofeleventh-rotary-step springs 858, to the winding of relay 828 of two-waytrunk circuit TWT2. Relay 828 is thereby operated to close a circuit foroperating relay of signal transmitter ST2. This circuit includes thefront contact of relay 829, back contact of relay 828, conductor 908,back contact of relay 909, conductor 9|0, and winding of relay 100. Theoperation of relay 100 causes the transmission of an answering or of!-hook supervisory signal consisting of an impulse of current of frequencyF4 followed by an impulse of current of frequency F5, the variousoperations of the signal transmitter ST2 being similar to those of .thesignal transmitter STI in sending a seizure signal as hereinbeforedescribed. The signal receiver SRS responds to the olf-hook supervisorysignal in a similar manner to that in which the signal receiver SR2responds to a seizure signal as hereinbefore described. The operation ofrelay 32d is effective to connect ground through conductor l2-5, a backcontact of relay A69, conductor 365, lower front contact of relay m6 ofoutgoing trunk circuit OTI to the winding of supervisory relay H35 andthe winding of relay l i9. Relay I i9 operates and locks to ground onconductor fc3. The operation of relay H35 disconnects ground from thelower, low resistance winding of relay it thereby to extinguish thesupervisory lamp SL associated With plug CPI t indicate that the callhas been answered. The operation of relay m connects ground to conductorll to operate relay 460 of signal receiver SR2. Relay 45B disconnectsground from conductor 454, disconnects conductor 425 fromconductor 465,connects conductor 425 to conductor 463, and connects conductor 665 toground at the back contact of relay 334.

lf the answering operator disconnects from jack 5l@ before the callingoperator releases the connection, relay lett is released as described inthe aforementioned King et al. patent. The release of relay ill causesthe release of relay 82S; and the release of relay 329 causes therelease of relay 'lull of signal transmitter ST2. At the time that relayI releases, relays (i-, 526, 65d, elfi, G'Sl, ill, teil, till, lli),75l), 769, H5 and 'i90 are operated. The release of relay Fll causes thetransmission of an on-hook supervisory signal consisting of an impulseof current of frequency F-'l followed by an impulse of current offrequency F5 followed by an impulse of current of frequency F5 in thesame manner that the first impulse of a train of dial impulses istransmitted by the signal transmitter ST5 as hereinbefore described. Thetransmission of current of frequency Ft is continued, since relay S25)is locked under the control of relay 690, until relay Sill? is operatedas hereinafter described. The signal receiver SRI responds to theon-hook signal in the same manner that the signal receiver SR2 respondsto the first dial impulse of a train, the operation of relay E34 beingeffective to disconnect ground from conductor it to release relay 65 andthereby effect the lighting of the supervisory lamp SL. Relays IIS and46%) remain operated until the connection is released as hereinafterdescribed. The operation of rela'y .1355 also connects ground toconductor 463 to maintain the operative energization of the lowerwinding of relay 126. As long as relay #12S is held operated, thecut-off relay 2M cannot reoperate and relay SI5 is held operated by itsmiddle winding.

If the calling operator wishes to recall the answering operator, relayi953 is momentarily released responsive to the momentary operation ofthe ringing key of cord CDI in the manner described in theaforementioned King et al. patent. The momentary release of relay i139causes a like release of relay 3M whereby a rering signal istransmitted. The rerng signal is transmitted in similar manner to thatin which the signal corresponding to the digit i is transmitted; and itconsists of an impulse of current of frequency Fl followed by an impulseof current of frequency F2 followed by an impulse of current offrequency F3 followed by an impulse of current of frequency F2. ,Inresponseto the rering sig-.-

operated.

nal, the signal receiver SR2 momentarily disconnects ground fromconductor 565 thereby causing the momentary release of relays 825 andH305 to recall the answering operator in the manner described in detailin the aforementioned King et al. patent.

When the calling operator disconnects' plug CPI from jack J I, theoutgoing trunk circuit is restored to normal in the manner described inthe aforementioned King et al. patent except that relay IIS remainsoperated while the disconnect signal is being transmitted. The releaseof relay H39 causes the release of relay 380 of signal transmitter SI.The release of relay Sell effects the transmission of an impulse ofcurrent of frequency Fi followed by an impulse of current of frequencyF2, followed by an impulse of current of frequency F3 in the same manneras upon release of relay 363 in response to the iirst impulse of a trainof dial impulses as hereinbefore described. But relay Sfl does notreoperate as it does at the end of a dial impulse; and therefore relay375 is not reoperated, relay 34H3 remains operated, and relay 353 is notre- The contained release of relay 375 causes the successive release ofrelays 210, 21| and 220, the transmission of current of frequency F3being continued until relay 220 releases, about l second after therelease of relay 350. If relay 230 is not operated responsive to adisconnect acknowledgment signal incoming over line rZfLI2 ashereinafter described before relay 22S releases, the release of relay229 disconnects source G3 from transformer 29S to end the transmissionof current of frequency F3 and connects source G2 to transformer 29d toinitiate the transmission of current of frequency F2. The release ofrelay 22H also closes a circuit for energizing the lower winding ofrelay 325], this circuit being traced through a front contact of relaySill, conductor 242, inner lower back contact of relay 2453, uppei-mostback contact of relay 220, inner upper front contact of relay 36D, toground at the front contact of relay 33D. Relay 32() is thereby reileased, causing the release of relay 325 whereby source G2 isdisconnected from transformer 29 to end the transmission of current offrequency F2; and source GI is connected to transformer 209 to initiatethe transmission of current of frequency Fl over line TLIZ, thisconnection being traced through conductor 3i5, lowermost front contactof relay 3M), upper back contact of relay 3&0, back contact of relay325, conductor ZI I, and a back contact of relay Zl to transformer 2l9.The release of relay 329 also causes the energization of the upperwinding of relay 330, thereby to effect the immediate release of relay339. The release of relay 33) causes the deenergization of the lowerWinding and reoperation of relay 329 after an interval of about 0.20second. The operation of relay 32B is followed by the reoperation ofrelay 325 and the reoperation of relay 33B at the end of a furtherinterval of about .020 second. The reoperation of relay 325 is effectiveto disconnect source GI and connect source G2 to transformer 209. Relays32, 325 and 332i are in this manner alternately released and reoperatedto alternately transmit impulses of current of frequencies F2 and FIover toll line TLI2 until a disconnect acknowledgment signal is receivedby the signal receiver SRI as hereinafter described.

This disconnect signal transmitted by the signal transmitter STI effectsthe successive operations of relays 5M, 524 and 534 responsiveto theimpulses of current of frequencies FI, F2 and F3; and effects thealternate operation of relays 524 and 5I4 responsive to the succeedingalternate impulses of frequencies F2 and FI. The release of relay 5I4 atthe end of the first impulse of frequency FI causes the operation ofrelay5I6; the operation of relay 524, responsive to the first impulse offrequency F2, causes the operation of relay 526; and the operation ofrelay 534, responsive to the impulseof frequency F3, causes relays 5I6and 526 to be held operated and disconnects ground from conductor 565,thereby causing the release of relays 825 and |085. The release of relayH165 causes a disconnect signal to be given to the answering operator ifthe answering cord has not been disconnected from jack JIU. If there areno interference currents, the impulse of current of frequency F3maintains the energization of relay 534 and the release of relay 825 for`a, long enough interval to cause the release of relay 821. The releaseof relay 821 causes the release of relay 823 and disconnects ground fromconductor 830 thereby to cause the release of relay 853 of selector TSZ.The release of relay 853 causes the operation of release magnet 869 toeffect the return of selector TS2 to normal in usual and well knownmanner. The release of relay I005 and the disconnection of the answeringcord from jack J I8 restore the incoming trunk circuit to normal in themanner described in the aforementioned King et al. patent. Theaforementioned disconnection of ground from conductor 830 due to therelease of relay 821 causes the release of relay 560.

If at the time that relay 560 releases, the call has not yet beenanswered or has been answered and the answering operator hasdisconnected, relays 829 and 100 are normal; but if the call has beenanswered and the answering operator has not disconnected, relays 829 and160 are operated and in this case the aforementioned release of relay821 causes the successive release of relays 828 and 100. Assume firstthat the call is one which has not been answered and that the release ofrelay 560 results from the impulse of current of frequency F3 of theaforementioned disconnect signal. In this case relays 690, 69|, 650 and680 are operated, but all other relays of the signal transmitter ST2 arenormal at the time that relay 560 releases. The release of relay 566disconnects ground from conductor 562; but, since relay 534 is operated,the release of relay 550 does not connect ground to conductor 564 sothat the winding of relay 640 is no longer short-circuited and thiswinding is operatively energized in series with the winding of relay658. The operation of relay 648 opens the circuit for operating relay680. Relay 580 is slow in releasing and remains operated for about .040second after relay 640 operates. The aforementioned disconnection ofground from conductor 908 causes the deenergization of the lower windingof relay 690; but relay 690 is slow in releasing and the operation ofrelay 640 closes a locking circuit through the upper winding of relay680 to hold relay 698 operated. Relay 640 closes a circuit for operatingrelay 160, closes a .circuit including the front contact of relay 680for operating relay-1I8, and connects source G4 through conductor 643,back contact of relay 125, conductor 6II and inner back contact of relayGID, to the right winding of transformer 669, thereby initiating thetransmission of current of frequency F4 over line TLI2 to the callingtoll oiiice. Relays 160 and 1I0 lock through conductor 633 and a backcontact of relay 630 to ground at a front contact of relay B50. Theoperation of relay 'IIQ opens the normally closed circuit for energizingthe lower winding of relay 120, the operation of relay 126 being delayedfor about .020 second by current charging condenser 12i. The operationof relay 120 closes a circuit for operatively energizing the lowerwinding of relay 125. The operation of relay disconnects transformer 609from source G4 to end the transmission of current of frequency F4 andconnects transformer 609 to source G5 to initiate the transmission ofcurrent of frequency F5. The connection between transformer 609 andsource G5 includes the inner back contact of relay Gl, conductor 6I I,front contact of relay 125, lower back contact of relay 150, conductor152, a back contact of relay 620, and another back contact of relay SIU.The aforementioned operation of relay 120 opens the normally closedcircuit through the upper winding of relay 130, the resulting operationof relay 130 being delayed for about .020 second by current chargingcondenser 122. The operationvof relay 130 closes a circuit forenergizing the lower winding of relay 120, this circuit being tracedthrough the middle lower front contact of relay 1I0, conductor 642,inner lower front contact of relay 640, conductor 163, inner upper frontcontact of relay and the front contact of relay 130. Relay 120 isthereby released, causing the release of relays 125 and 13u. The releaseof relay 125 disconnects source G5 from transformer 609 ending thetransmission of current of frequency F5 and reconnects source G4 totransformer 608 to initiate the transmission of current of frequency F4.Relays 126, 125 and 130 are thus cyclically operated and released toeffect the transmission of alternate impulses of current of frequenciesF4 and F5 as a disconnect acknowledgement signal. When relay 534releases at the end of the impulse of current of frequency F3 of thedisconnect signal transmitted by the signal transmitter STI, ground isreconnected to conductor t 564 thereby short-circuiting the winding andcausing the release of relay 648. Since ground has been disconnectedfrom conductor 830 due to the release of relay 821 of two-way trunkcircuit TWTZ, the release of relay 640 causes the release of relay 690.The release of relay 680 causes the release of relays 69| and 650. Therelease of relay 650 prevents the operation of relay 586 and, sincerelay 68| is slow in releasing, causes the release of relays 110 and160, relay 'i60 being slow in releasing. With relay 646 normal, therelease of relay 1I0 disconnects source G4 from transformer 608; and,with both of relays Illl and 'IIO normal, relays 120 and 125 are normalso that source G5 is disconnected `from transformer 609; and, therefore,the transmission of the disconnect acknowledgement signal is ended. Therelease of relay 1I!) closes the normally closed circuit for operatingthe cut-off relay 604.

Assume now that the call is one which has been answered, that theanswering operator has not disconnected and that relay 560 is releasedand selector TS2 restored to normal in response to the impulse ofcurrent of frequency F3 of the aforementioned disconnect signal. At thetime that relay 568 releases, relays 620, 650, 810, 61|,68ll,69D,69I,1Il8,1I0, 150, 160, and 115 of signal transmitter ST2 areoperated and the other relays of this transmitter are normal. Therelease of relay-853 of selector :TS-2 when-:relayg321 of two-'way trunkcircuit TWTZ releases, 'causes the successive release of relay 829 oftrunk circuit .TWT2 and relay 'l of signal transmitter ST2. The releaseof relay 56) disconnects ground from conductor 552 thereby causing theoperativeenergization of the winding of relay 640 in series With thewinding of relay 550. Although the release of relay 821 of two-way trunkcircuit TWT2 effects the deenergization of the lower winding of relay555. relay 69] is slow in releasing and the operation of relay 649closes a locking circuit through the upper winding of relay 590fto holdrelay 695 operated. The release of relay 15D causes the release of relay'V15y operation of relay 180, release of relay 195, operation of relay110, operation of relay 140 and release of relay 150 in the same manneras above described when relay 180 releases to effect the transmission ofan onhook signal to the calling operator. The release of relay T55causes the release of relays 610 and @il but relay 520 is held operatedby relay 690. The operation of relay 640 immediately connects source Gilto the back contact of relay 125 thereby initiating the transmission ofcurrent of frequency F4 over line TLIZ to the calling office. Therelease of relay 100 and operation of relay 640 open the circuit forenergizing the lower 'winding of relay 120, thereby causing the delayedoperation of relay 120. When relay 120 operates, it causes the immediateoperation of relay 125 and delayed operation of relay 130. The operationof relay 125 ends the transmission of Acurrent of frequency F4 andinitiates the transmission of current of frequency F5, source G beingconnected through the uppermost front Contact of relay B40, inner upperfr ont contact of relay 620, conductor 152, and lower back contact ofrelay 150 to the front contact of relay 125. Relays 120, 125 and 130 arecyclically operated and released to elect the transmission of alternateimpulses of frequencies F4 and F5 as a disconneet acknowledgment signal.When-the impulse of current of frequency F3 incoming over line 'I'Ll2ends, the release of relay 534 connects ground to conductor 564 torelease relay 640. The release of relay 640 causes the release of relay690 and the release of relay 690 causes the release of relays 62D, 69|and 650. The release of relay 650 causes the release of relays 1|0 and160, thereby ending the transmission of the disconnect vacknowledgmentsignal. The-release of relay 110 also causes the reoperation of cut-offrelay 604.

Assume next that the call is one-which has been answered, that theanswering operator has disconnected and that relay 560 releasesresponsive to the impulse of current of frequency F3 of theaforementioned disconnect signal. In this case relays 525, 65D, 680,590, 69|, 1H), 120, 125, 130, 140 and 160 of signal transmitter ST2 areoperated at the time that relay 560 releases. Relays G'l, 51| and 665will also be operated in case current of frequency F5 is beingtransmitted by signal transmitter ST2 as part of an on-hook signal atthe time that relay 560 releases. IThe cut-off relay E04 will bereleased while an onhook signal is being transmitted by signaltransmitter ST2 and will be released due to the operation of relay 525during receipt of the disconnect signal by signal receiver SR2. Therelease of relay 550 disconnects ground from conductor 562 to cause theoperation of relay B40. The operation of relay 640 disconnects source GSfrom the front contact of relay 125, connects source G5 to the frontcontact'of relay 125 and connects source G4 to the back contact of relay125. Thus the operation of relay 64E) ends the transmissionof current offrequency F6 and initiates the trans'- mission of current of frequencyF5. The operation of relay 640 also closes the circuit for energizingthe lower winding of relay 12!) to cause the release of relay 120. Therelease of-re1ay'12 causes the release of relays and 139; and thesethree relays are cyclically operated andre- -leased to transmitalternate impulses of frequencies F4 and F5 over line TLI2 to thecalling officev as adisconnect acknowledgmentsig'- nal until relay 640releases responsive to the ending of the impulse vof current offrequency'F incoming over line TLI2 from the calling office.'

Assume now that the impulse of current of frequency F3 incoming overline' TLI2 as a disconnect signal is ineffective to cause the operationof relay 534 for a long enough interval to effect l the release of relay821 of two-way trunk circuit TWTZ. In this case relay 56B is notreleased and relays 524 and 5|4 are alternately operated vresponsive tothe alternate impulses of frequencies F2 and FI transmitted by signaltransmitter STI as hereinbefore described. Relays 525 and 5|6 are heldoperated during receipt of these impulses. The operation of relay 5M bythe first impulse of frequency FI, following the impulse of frequencyF3, opens the short-circuit 'across the upper winding of relay 550 andthis winding is energized in series with resistor '552,"cof1ducto'r'553'and the front contact of 'relay 5 l5; Theoperation of relay 550opens the short-circuit" across the winding of relay 555, therebycausing theop'er'- ation of relay 555. The operation of relaye555co'nnects ground to conductor- 556 to operate relay 630 of signaltransmitter ST2. When relay "5M releases, it again short-circuits the`winding of relay 550, and relay 550'releases; butrelay 555 remainsoperated as long asv relay 5 l 6 is held operated responsive vtoincoming signal 4current impulses. The operation of're'lay 635,0pens sthe short-circuit across the winding of relay 54.5 to cause theoperation of relay 64I) in servies with relay 650. The operationofrelays 64D and 535 connect the locking winding of relay 595 throughconductor 562 to ground at a front contact of relay 560. The operationof relay 640 causes the' transmission of a disconnect acknowledgmentsignal in the manner described above.

When the alternate impulses of current of frequencies F4 and F5, whichare transmitted. by the signal transmitter ST2, as .a disconnectacknowledgement signal are received bysignal receiver SRI at the callingend oftoll line TLIZ, relays 4|4 and 424 are alternately operated. Theoperation of relays 4|4 and -424 causes corresponding operation ofrelays 4|6 and 42E in the same manner that relays 5l6 and 52'@` ofsignal receiver SR2 are operated responsive to incoming signals ashereinbefore described. The operation of relay 426 holds relay 4| 6 andthe next succeeding operation of relay 4|4 opens the shortcircuit acrossthe upper winding of relay 45|)` to cause the operation of relay 450.The operation of relay 450 opens the short-circuit across the winding ofrelay 455 so that relay 455 operates;-

When relay 4l4 releases, relay 450` releases due to the short-circuitingof its upper winding but relay 455 remains operated since thereleaseofi. relay 45D does not again close the short-circumacross thewinding of relay 455. The operation of relay 455 connects ground4 toconductor 455,to

operate relayA 230. The operation -of relay 230 opens the lockingcircuit for relays 3H] and 360. Relay 3|0 releases but relay 360 remainsoperated if the operation of relay 230 occurs before relay 220 hasreleased. The release of relay 3|!) closes the normally closed circuitfor energizing the lower winding of relay 320 thereby effectingtherelease of relay 320. The release of relay 320 causes the release ofrelays 325 and 330. The release of relay 325 disconnects source VG3 fromtransformer 209 ending the transmission of current of frequency F3. Whenrelay 220 releases, the cut-off relay `204 is reoperated.

If the disconnect acknowledgment signal is not received by signalreceiver ASRI before relay 220 of signal transmitter STI has released,the release of relay 220 ends the transmission of current of frequencyF3 and initiates the transmission of current of frequency F2; andimpulses of current of frequencies F2 and Fi are alternately transmitted.as above described until the disconnect acknowledgment signal isreceived. When relay 231)v is operated, responsive to the disconnectacknowledgment signal, it closes a circuit for operating relay 2|0; thiscircuit is traced through the outer lower back contact of relay 240,tinner upper back contact of relay 220, upper front contact of relay 230,Vand, back contacts of relays 29| and 290. The operation of relay 2|0endsv the transmission of current of either of frequencies FI and F2 andconnects source G3 to transformer 209 to .effect thetransmissien of; enimpulse 0f current of: Y.frequency F3 over toll line TML The operationof relay 2|0 also connects ground to conductorl |08 to mark` the zassociated trunk circuits busy until, disconnection is completed. Theaforementioned operation of relayy 230 disconnects ground from conductor233 to release relay 3H) and thereby end the cyclic .operation .ofrelays 320, 325 and 330. The transmission of current of frequency F3continues as long as the disconnect acknowledgment signal is beingreceived from the signal transmitter ST2.

The current of frequency F3, transmitted by signal transmitter STIimmediately following the transmission of current of frequency Fl or VF2responsive to the disconnect acknowledgment signal received by signalreceiver SRI causes the.

operation of relay 534of signal receiver SR2 while the signaltransmitter ST2 is transmitting alternate impulses. of current offrequencies F4 and E5'. The operation of relay 5,34 .disconnects groundfrom conductor 56,5 to cause therelease of the connection in the samemanner that it is i effected responsive to the first impulse of currentof frequency E3 transmitted as a result of the release of the connectionby the calling operator as hereinbefore described. The release of relay560 disconnects ground from conductor 562 thereby opening the circuitthrough the locking winding of relay 680. The release ofv relay 690causes the release of-relaysBSl, 650. 640-and 620. The release of relay640 causes the release of relays 110i and 160 thereby to endgthetransmission of the disconnect acknowledgment signal as hereinbeforedescribed. When the disconnect-acknowledgement signal is ended, relays415 and 426 of Vsignal receiver SR-I release. The release of relay 4-l6causes the successive release of relays 455, 230 and 2M). The release ofrelay l2I0 ends the transmission of current of vfrequency F3. Therelease of relay 426 causes the reoperation of cut-off relay 204.

Whatl is claimed is:

1. In a signaling system in which the transmission of a signal iseffected by an impulse of current of a first signaling frequencyfollowed by an impulse of current of a second signaling frequencyfollowed by an impulse of current of a third signaling frequency, asignal receiver for responding to said signal, said receiver comprisingthree relays, means rendering a first one of said relays responsive onlyto current of the first signaling frequency, means rendering a secondone of said relays responsive only to current of the second signalingfrequency, means rendering the third of said relays responsive only tocurrent of the third signaling frequency, means rendering said secondrelay operative for a predeterminedv interval 0f time following theoperation and release of said first relay, means rendering said thirdrelay operative for a predetermined interval of time following theoperation and release of said second relay, and signal means controlledby said second relay.

2. In a signaling system in which the transmission of a signal iseffected by an impulse of current of a first signaling frequencyfollowed by an impulse of current of a second signaling frequencyfollowed by an impulse of current of a third signaling frequency, asignal receiver ior responding to said signal, said receiver comprisingthree relays, means rendering a first one of said relays responsive onlyto current of the first. signaling frequency, means rendering a secondone ofsaid relays responsive only to current of the second signalingfrequency, means rendering the third .of said relays responsive only tocurrent of the third signaling frequency. means rendering said secondrelay operative for a predetermined inteiyal of time following theoperation and release of said first relay. means rendering said thirdrelay operative for a predeterminedv interval of time ,following theoperation and release of said second relay, and signal means controlledby said third relay.

3. In a signaling system in which the transmission of a' signal iseffected by an impulse of current of a first signaling frequencyfollowed by. an impulse of current of a second signaling frequencyfollowed by an impulse of current of a third signaling frequency, asignal receiver foi` responding to said signal, said receiver comprisingthree relays, means rendering a first one of said relays responsive onlyto current of the rst signaling frequency, means rendering a second oneof sai'd relays responsive only to current of the second signalingfrequency, means rendering the third of said relays responsive only tocurrent of the third signaling frequency, means rendering said secondrelay operative for apredetermined interval of time following theoperation and release of said first relay, means rendering said thirdrelay operative for a predetermined interval of time 'following' theoperation and release of said second relay, and signal means controlledby said second and third relays.

4. In a signaling system in which the transmission of each signal iseffected by a plurality of impulses of alternating current insuccession, each impulse consisting of current of only one of threedifferent signaling frequencies, a signal receiver and signal meanscontrolled by said signal receiver', said signal receiver comprising arst relay, means rendering said first relay responsive only to currentof a first signaling frequency, a second relay, means rendering saidsecond relay responsive only to current of a sec- Vond signalingfrequency, a third relay, means `a predetermined interval of timeresponsive to the operation and release of said rst relay, meansnormally biasing said second relay to prevent its operation, means fordecreasing the bias of said second relay to render said second relayoperative While said fourth relay is operated, a fth relay, means foroperating 'said fifth relay responsive to the operation of said secondrelay, means normally biasing said third relay to prevent its operation,means for operatively energizing said fourth relay and for decreasingthe bias of said third relay to render said third relay operative whilesaid fth relay is operated, means for operating said signal meansresponsive to the operation of said second relay, and means formaintaining the operation of said signal means under the control of saidthird relay.

5. In a signaling system in Which the transmission of each signal iseffected by a plurality of impulses of alternating current insuccession, each impulse consisting of current of only one of threedifferent signaling frequencies, a signal receiver and signal meanscontrolled by said signal receiver, said signal receiver comprising arst relay, means rendering said first relay responsive only to currentof a first signaling frequency, a second relay, means rendering saidsecond relay responsive only to current of a second signaling frequency,a third relay, means rendering said third relay responsive only tocurrent of the third signaling frequency, a fourth relay, means foroperating said fourth relay for a predetermined interval of timeresponsive to the operation and release of said rst relay, meansnormally biasing said second relay to prevent its operation, means fordecreasing the bias of said second relay to render said second relayoperative While said fourth relay is operated, a fifth relay, means foroperating said fth relay responsive to the operation of said secondrelay, means effective to maintain the operaf-ive energization of saidfifth relay for a predetermined interval of time after said second relayreleases, means normally biasing said third relay to prevent itsoperation, means for operatively energizing said fourth relay and fordecreasing the bias of said third relay to render said third relayoperative While said fth relay is operated, means for operating saidsignal means responsive to the operation of said 4second relay, andmeans for maintaining the operation of said signal means under thecontrol of said third relay.

6. In a signaling system in which the transmission of each signal iseffected by a plurality of impulses of alternating current insuccession, each impulse consisting of current of only one of threedifferent signaling frequencies, a signal receiver and signal meanscontrolled by said signal receiver, said signal receiver comprising afirst relay, means rendering said rst relay responsive only to currentof a first signaling frequency, a second relay, means rendering saidsecond relay responsive only to current of a second signaling frequency.a third relay, means rendering said third relay responsive only tccurrent of the third signaling frequency, a fourth relay, means foroperating said fourth relay for a predetermined interval of timeresponsive to the operation and release of said first relay, meansnormally biasing said second relay to prevent its operation', means fordecreasing the'bias of said second relay to render said second relayoperative While said fourth relay is operated, a fifth relay, means foroperating said fifth relay responsive to the operation of said secondrelay, means normally biasing said third relay to prevent its operation,means for operatively energizing said fourth relay and for decreasingthe bias of said third relay to render said third relay operative Whilesaid fth relay is operated, means for operating said signal meansresponsive to the operation of said second relay, means for maintainingthe operation of said signal means under the control of said thirdrelay, and means for operatively energizing said fifth relay while saidthird relay is operated and for maintaining the energization of saidfifth relay for a predetermined .interval of time after the release ofsaid third relay.

7. In a signaling system in Which the transmission of each signal iseffected by a plurality of impulses of alternating current insuccession, each impulse consisting of current of only one of threedifferent signaling frequencies, a signal receiver and signal meanscontrolled by said signal receiver, said signal receiver comprising afirst relay, means rendering said rst relay responsive only to currentof a rst signaling frequency, a second relay, means rendering saidsecond relay responsive only to current of a second signaling frequency,a third relay, means rendering said third relay responsive only tocurrent of the third signaling frequency, a fourth relay, means foroperating said fourth relay for a predetermined interval of timeresponsive to the operation and release of said first relay, meansnormally biasing said second relay to prevent its operation, means fordecreasing the bias of said second relay to render said second relayoperative While said fourth relay is operated, a fth relay, means foroperating said fifth relay responsive to the operation of said secondrelay, means effective to maintain the operative energization of saidfifth relay for a predetermined interval of time after said second relayreleases, means normally biasing said third relay to prevent itsoperation, means for operatively energizing said fourth relay and fordecreasing the bias of said third relay to render said third relayoperative while said fifth relay is operated, means for operating saidsignal means responsive to the operation of said second relay, means formaintaining the operation of said signal means under the control of saidthird relay, and means for operatively energizing said fifth relay Whilesaid third relay is operated and for maintaining the energization ofsaid fifth relay for a predetermined interval of time after the releaseof said third relay.

8. In a signaling system in which the transmission of each signal iseffectedV by a plurality of impulses of alternating current insuccession, each impulse consisting of current of only one of threedifferent signaling frequencies, a signal receiver and signal meanscontrolled by said signal receiver, said signal receiver comprising afirst relay, means rendering said rst relay responsive only to currentof a first signaling frequency, a second relay, means rendering saidsecond relay responsive only to current of a second signaling frequency,a third relay, means rendering Asaid third relay responsive only tocurrent of the third signaling frequency, a fourth relay, means foroperating said fourth relay for a predetermined interval of timeresponsive to the operation and release of said first relay, meansnormally bias-

